Printing technologies are typically broadly classified as technologies that require an image carrier or a master (a printing plate), referred to as conventional or analog printing technologies, and the so called non-impact printing (NIP) technologies which do not require a printing plate, also know as digital printing. The major printing processes are also distinguished by the method of image transfer and by the general type of image carrier employed. Depending upon the process, the printed image is transferred to the substrate either directly or indirectly. In direct printing, the image is transferred directly from the image carrier to the substrate. Examples of direct printing are gravure, flexography, screen printing and letterpress printing processes. In indirect, or offset, printing, the image is first transferred from the image carrier to the blanket cylinder and then to the substrate.
All the conventional technologies use ink (colorant material) that is a liquid. Conventional printing includes screen printing, letterpress (including flexographic printing), lithography (offset, waterless offset) and gravure. NIP technologies include electrophotography (dry toner and liquid toner), ionography, magnetography (magnetic toner), inkjet (continuous uses liquid ink, drop on demand uses liquid or hot melt ink), thermography (sublimation, transfer) and silver halide photography. The printing inks or marking materials used for each of these printing technologies consists of colorants (pigments, dyes), vehicles (binders), additives and carrier substances (solvents). Depending on the printing process, inks have largely variable flow properties from extremely thin (less viscous, used in ink jet and gravure printing) through highly viscous (offset, letterpress) up to dry powder.
Marking technologies in common use in the printing world develop color by the deposition of the colorant material directly on the surface of the media. Therefore this is normally an extrinsic process of development of color and the required density for the print. This contrasts distinctly with the photographic process, in which the receiver or the specially sensitized media has built-in chemistry to develop the appropriate colors and densities from within, which are intrinsic to the media. A limitation of the extrinsic image formation process is that the image physicals are harder to control, because the colorant materials are exposed at the surface.
All of the above printing technologies can be classified as either analog or digital depending on the printing method as described earlier. Analog printing reproduces images with like images by employing analogous image transfer from a master image. Unlike analog printing technology that use stencils or plates containing full sized images, digital printing approaches assemble each image printed from a complex of numbers and mathematical formulas. They configure images from a matrix of dots or pixels and use digitally controlled deposition of ink, toner or exposure to electromagnetic energy, such as light, to reproduce images.
The structure and components of ink or colorant materials is determined by ink transfer mechanism and type of drying/fixing of the ink on the substrate/print medium. Since the technologies are so varied, typically printing media are tailored for each of the technologies. In all of the above printing methods high surface gloss is difficult to attain without an added foreign component in a second operation. Gloss is a measure of light reflectance from the surface. Usually, it is achieved by either lamination of the print with a sheet of plastic, or by coating over the print with either an aqueous or UV curable varnish. Gloss is sought because it provides printed products with a snappy overall attention-getting look, provides a greater depth of color and chromaticity and exhibits a higher gamut. For example, a glossy black appears to be blacker than a matte black and a glossy red darker and more intense than a matte red.
The production of near photographic quality images using commercial printing technologies such as, dry and liquid electrostatic printing and offset printing is highly desirable. It is even more desirable to produce such images on media that render the print with the look and feel of a typical photographic print produced with silver halide imaging technology, such as the degree and uniformity of glossiness, stiffness and opacity, and high resolution and sharpness with corresponding low grain appearance. While there are a variety of substrates available today for the different printing modalities, there is no one universal media that can give the same look and feel no matter what the printing method. With hybrid printing jobs that combine offset and digital jobs there is in need for media that can run on both types of machines and produce the same look and feel and a high degree of gloss.
When printing calendars, greeting cards and post cards, it is particularly desirable that the printed backside of the output can be marked with a writing instrument such as a ball point pen. U.S. Patent Application 2002/0037176 A1 discloses a receiver sheet provided with a special coating on the backside to enable writablilty. Similarly U.S. Pat. No. 5,658,677 discloses a one-sided image carrier where the backside is coated with silica rich coatings to impart writability. These constructions, while very useful, do not lend themselves to the manufacture of two-sided sheets where both sides can be imaged using different printing modalities and where one side can be selectively glossed.